Broadcast on all Thai TV networks, the statement said: “Even though she has relinquished her royal titles in writing, she maintained her status and carried herself as a member of the Chakri dynasty.

“Involvement of a high-ranking member of the royal family in politics, in whatever way, is considered an act that defies the nation’s traditions, customs and culture, and therefore is considered extremely inappropriate.”

The statement cited a passage of the constitution that says the monarchy should maintain political neutrality.

Next month’s vote is being closely watched as the first chance for Thailand to return to democracy after five years under military rule.

Analysts say the king’s intervention would in any case have been likely to have led to the election commission disqualifying her from the 24 March election.

The BBC’s Jonathan Head said the move to nominate the princess looks like a grave miscalculation, one likely to weaken the pro-Shinawatra faction trying to push the military out of politics.

What has the princess herself said?

Hours earlier, Princess Ubolratana defended her decision to run for office.

In an Instagram post, she reiterated that she had relinquished all her royal titles and now lived as a commoner.

She said she wanted to exercise her rights as an ordinary citizen by offering her candidacy for prime minister. She said she would work with all sincerity and determination for the prosperity of all Thais.

Shortly after the king’s statement she posted again without addressing the issue directly, simply thanking Thais for their support and saying that she wanted Thailand to “move forward and become admired and accepted by the international community”.

Who is Princess Ubolratana Mahidol?

Born in 1951, Princess Ubolratana Rajakanya Sirivadhana Barnavadi is the oldest child of Thailand’s late King Bhumibol Adulyadej. He died in 2016.

She attended the Massachusetts Institute of Technology and after marrying an American in 1972 she gave up her royal title. After her divorce she returned to Thailand in 2001 and once again started participating in royal life.

The princess engages actively in social media and has also starred in several Thai movies.

She has three children, one of whom died in the 2004 Asian tsunami. The other two now also live in Thailand.

Why is the election important?

It will be the first vote since current Prime Minister Prayuth Chan-ocha took power in 2014, overthrowing the democratic government and ousting ex-PM Yingluck Shinawatra, the younger sister of Mr Thaksin.

Both Mr Thaksin and his sister live in self-imposed exile but remain a powerful force in Thai politics, with many in the country remaining loyal to them.

In 2016, Thais voted to approve a new constitution created by the country’s military leaders, which was designed to perpetuate military influence and block Mr Thaksin’s allies from winning another election.

But the princess aligning herself with a party allied with Mr Thaksin threatened those plans, correspondents say.